The present invention relates to a glass substrate, which is formed with a concave portion, such as a V-groove portion and a countersunk portion, with a high surface accuracy on the surface of the glass substrate, and a two-stage forming method for the same.
As a fiber arraying part, there have been known a fiber array for the connection of a fiber to other optical parts and an MT connector for connecting fibers to each other. These optical parts for connection are manufactured by using a glass substrate having a fiber fixing groove. As one example, FIG. 5 shows the shape of the glass substrate having a groove in the surface thereof, what we call a V-grooved substrate.
As a method for molding a V-grooved glass substrate 3 having a V-groove 1 and a countersunk portion 2, which is a coated fiber storage portion, shown in FIG. 5, a reheat press is thought to be effective. The reheat press is a method in which molten glass is inserted into a mold, and a molded body roughly press molded into a desired shape or a body worked into a desired shape by grinding a glass material is press molded precisely by using a die while being reheated. Although a molded product having a V-groove only can be formed by even grinding, the countersunk shape as shown in FIG. 5 cannot be obtained by grinding. Therefore, the reheat press, which can precisely transfer the molding surface of die, is effective. Also, other than the reheat press, ultrasonic machining may be considered. However, this method has a problem in that chipping occurs on the machined surface when the roughness of machined surface has a P.V value not lower than 10 .mu.m.
For the reheat press, since the accuracy of a die is reflected in the shape of molded surface, how the accuracy of a die surface is improved is an important problem. A die 4 for molding a V-grooved glass substrate shown in FIG. 5 is as shown in FIG. 6.
The shape of a convex portion 5 for the V-groove of this die 4 requires an accuracy within 1 .mu.m on the surface thereof, so that it is difficult to achieve this accuracy unless grinding is performed. However, since in grinding, a grinding stone hits on the convex portion 5 for V-groove and a convex portion 6 for countersunk portion, it is actually difficult to manufacture this die by grinding.
A method in which a die is divided into several parts and these parts are assembled into one die, for example, a method in which a die part including the V-groove portion and a die part including a countersunk portion are manufactured separately and these two die parts are fixed with screws, has a high machining accuracy because each die part can be ground. However, as shown in FIG. 6, a gap of several micrometers is produced at a connecting portion 7. Therefore, if reheat press is performed by using this die 4, convex wall shaped burrs are produced on the molded product. If this burr is present between the V-groove and the countersunk portion, when a fiber core is placed, the fiber core is undesirably damaged.
Further, since misalignment easily occurs between the top surface of the V-groove portion and the top surface of the countersunk portion, the positional relationship between the fiber and the countersunk portion is shifted. Even if the gap is filled by welding of the connecting portion, the gap is reproduced by wear at the time of reheat press.
If the whole shape is manufactured by electrical discharge machining, the surface roughness of die becomes 10 .mu.m and larger. Even if this surface is subjected to mirror polishing, the undulation of surface of about 5 .mu.m is produced.
For the conventional optical fiber array, the boundary between the V-groove portion and the countersunk portion of the V-grooved glass substrate is as shown in FIG. 7. That is, since the countersunk portion 2 is formed by grinding, an edge is formed at a V-groove end portion 14, which is the boundary between the V-groove portion 1 and the countersunk portion 2. If the fiber core 15 hits on this edge, there is a possibility of the fiber core 15 being damaged and broken. Also, because of grinding, the surface roughness is also about 1 .mu.m in Rmax, or about 0.2 .mu.m or more in Ra.
On the other hand, formation of a concave portion in which the diameter of the bottom surface is larger than other diameters, for example, the diameter of the surface opening portion (hereinafter referred to as a dovetail-shaped concave portion for convenience) in the surface of the glass substrate is impossible to do by grinding, and even in press molding, mold release cannot be performed, so that molding is very difficult to do. As shown in FIG. 4, the dovetail-shaped concave portion 8 has an advantage in that if a resin 11 is poured into the dovetail-shaped concave portion 8 and is cured when two substrates 9 and 10 having such a concave portion are joined to each other, the resin 11 entering the bottom part of the concave portion is caught by a taper 12 (due a to so-called anchoring effect), so that separation is less liable to occur. For this reason, there is a demand for manufacturing a glass substrate having such a dovetail-shaped concave portion.